Network Working Group P. Hoffman
Internet-Draft ICANN
Intended status: Standards Track P. McManus
Expires: October 4, 2018 Mozilla
April 02, 2018
DNS Queries over HTTPSdraft-ietf-doh-dns-over-https-05
Abstract
This document describes how to run DNS service over HTTP using
https:// URIs.
[[ There is a repository for this draft at https://github.com/dohwg/draft-ietf-doh-dns-over-https [1] ]].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 4, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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The integration with HTTP provides a transport suitable for both
traditional DNS clients and native web applications seeking access to
the DNS.
Two primary uses cases were considered during this protocol's
development. They included preventing on-path devices from
interfering with DNS operations and allowing web applications to
access DNS information via existing browser APIs in a safe way
consistent with Cross Origin Resource Sharing (CORS) [CORS]. There
are certainly other uses for this work.
2. Terminology
A server that supports this protocol is called a "DNS API server" to
differentiate it from a "DNS server" (one that uses the regular DNS
protocol). Similarly, a client that supports this protocol is called
a "DNS API client".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14, RFC8174 [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Protocol Requirements
The protocol described here bases its design on the following
protocol requirements:
o The protocol must use normal HTTP semantics.
o The queries and responses must be able to be flexible enough to
express every DNS query that would normally be sent in DNS over
UDP (including queries and responses that use DNS extensions, but
not those that require multiple responses).
o The protocol must permit the addition of new formats for DNS
queries and responses.
o The protocol must ensure interoperability by specifying a single
format for requests and responses that is mandatory to implement.
That format must be able to support future modifications to the
DNS protocol including the inclusion of one or more EDNS options
(including those not yet defined).
o The protocol must use a secure transport that meets the
requirements for HTTPS.
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Internet-Draft DNS Queries over HTTPS April 20183.1. Non-requirements
o Supporting network-specific DNS64 [RFC6147]
o Supporting other network-specific inferences from plaintext DNS
queries
o Supporting insecure HTTP
4. The HTTP Request
A DNS API client encodes a single DNS query into an HTTP request
using either the HTTP GET or POST method and the other requirements
of this section. The DNS API server defines the URI used by the
request through the use of a URI Template [RFC6570]. Configuration
and discovery of the URI Template is done out of band from this
protocol.
The URI template defined in this document is processed without any
variables for requests using POST, and with the single variable "dns"
for requests using GET. The value of the dns parameter is the
content of the request (as described in Section 4.1), encoded with
base64url [RFC4648].
Future specifications for new media types MUST define the variables
used for URI Template processing with this protocol.
DNS API servers MUST implement both the POST and GET methods.
When using the POST method the DNS query is included as the message
body of the HTTP request and the Content-Type request header
indicates the media type of the message. POST-ed requests are
smaller than their GET equivalents.
Using the GET method is friendlier to many HTTP cache
implementations.
The DNS API client SHOULD include an HTTP "Accept" request header to
indicate what type of content can be understood in response.
Irrespective of the value of the Accept request header, the client
MUST be prepared to process "application/dns-udpwireformat"
Section 4.1 responses but MAY also process any other type it
receives.
In order to maximize cache friendliness, DNS API clients using media
formats that include DNS ID, such as application/dns-udpwireformat,
SHOULD use a DNS ID of 0 in every DNS request. HTTP correlates
request and response, thus eliminating the need for the ID in a media
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type such as application/dns-udpwireformat and the use of a varying
DNS ID can cause semantically equivalent DNS queries to be cached
separately.
DNS API clients can use HTTP/2 padding and compression in the same
way that other HTTP/2 clients use (or don't use) them.
4.1. DNS Wire Format
The data payload is the DNS on-the-wire format defined in [RFC1035].
The format is for DNS over UDP. Note that this is different than the
wire format used in [RFC7858]. Also note that while [RFC1035] says
"Messages carried by UDP are restricted to 512 bytes", that was later
updated by [RFC6891], and this protocol allows DNS on-the-wire format
payloads of any size.
When using the GET method, the data payload MUST be encoded with
base64url [RFC4648] and then provided as a variable named "dns" to
the URI Template expansion. Padding characters for base64url MUST
NOT be included.
When using the POST method, the data payload MUST NOT be encoded and
is used directly as the HTTP message body.
DNS API clients using the DNS wire format MAY have one or more EDNS
options [RFC6891] in the request.
The media type is "application/dns-udpwireformat".
4.2. Examples
These examples use HTTP/2 style formatting from [RFC7540].
These examples use a DNS API service with a URI Template of
"https://dnsserver.example.net/dns-query{?dns}" to resolve IN A
records.
The requests are represented as application/dns-udpwirefomat typed
bodies.
The first example request uses GET to request www.example.com
:method = GET
:scheme = https
:authority = dnsserver.example.net
:path = /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB
accept = application/dns-udpwireformat
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status codes do not contain DNS answers to the question in the
corresponding request. Some of these non-successful HTTP responses
(e.g., redirects or authentication failures) could allow clients to
make new requests to satisfy the original question.
Different response media types will provide more or less information
from a DNS response. For example, one response type might include
the information from the DNS header bytes while another might omit
it. The amount and type of information that a media type gives is
solely up to the format, and not defined in this protocol.
At the time this is published, the response types are works in
progress. The only response type defined in this document is
"application/dns-udpwireformat", but it is possible that other
response formats will be defined in the future.
The DNS response for "application/dns-udpwireformat" in Section 4.1
MAY have one or more EDNS options, depending on the extension
definition of the extensions given in the DNS request.
Each DNS request-response pair is matched to one HTTP request-
response pair. The responses may be processed and transported in any
order using HTTP's multi-streaming functionality ([RFC7540]
Section 5}).
The Answer section of a DNS response can contain zero or more RRsets.
(RRsets are defined in [RFC7719].) According to [RFC2181], each
resource record in an RRset has Time To Live (TTL) freshness
information. Different RRsets in the Answer section can have
different TTLs, although it is only possible for the HTTP response to
have a single freshness lifetime. The HTTP response freshness
lifetime ([RFC7234] Section 4.2) should be coordinated with the RRset
with the smallest TTL in the Answer section of the response.
Specifically, the HTTP freshness lifetime SHOULD be set to expire at
the same time any of the DNS resource records in the Answer section
reach a 0 TTL. The response freshness lifetime MUST NOT be greater
than that indicated by the DNS resoruce record with the smallest TTL
in the response.
If the DNS response has no records in the Answer section, and the DNS
response has an SOA record in the Authority section, the response
freshness lifetime MUST NOT be greater than the MINIMUM field from
that SOA record. (See [RFC2308].) Otherwise, the HTTP response MUST
set a freshness lifetime ([RFC7234] Section 4.2) of 0 by using a
mechanism such as "Cache-Control: no-cache" ([RFC7234]
Section 5.2.1.4).
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A DNS API client that receives a response without an explicit
freshness lifetime MUST NOT assign that response a heuristic
freshness ([RFC7234] Section 4.2.2.) greater than that indicated by
the DNS Record with the smallest TTL in the response.
A DNS API server MUST be able to process application/dns-
udpwireformat request messages.
A DNS API server SHOULD respond with HTTP status code 415
(Unsupported Media Type) upon receiving a media type it is unable to
process.
This document does not change the definition of any HTTP response
codes or otherwise proscribe their use.
5.1. Example
This is an example response for a query for the IN A records for
"www.example.com" with recursion turned on. The response bears one
record with an address of 192.0.2.1 and a TTL of 128 seconds.
:status = 200
content-type = application/dns-udpwireformat
content-length = 64
cache-control = max-age=128
<64 bytes represented by the following hex encoding>
00 00 81 80 00 01 00 01 00 00 00 00 03 77 77 77
07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00
01 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f
6d 00 00 01 00 01 00 00 00 80 00 04 C0 00 02 01
6. HTTP Integration
This protocol MUST be used with the https scheme URI [RFC7230].
6.1. Cache Interaction
A DNS API client may utilize a hierarchy of caches that include both
HTTP and DNS specific caches. HTTP cache entries may be bypassed
with HTTP mechanisms such as the "Cache-Control no-cache" directive;
however DNS caches do not have a similar mechanism.
A DOH response that was previously stored in an HTTP cache will
contain the [RFC7234] Age response header indicating the elapsed time
between when the entry was placed in the HTTP cache and the current
DOH response. DNS API clients should subtract this time from the DNS
TTL if they are re-sharing the information in a non HTTP context
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(e.g., their own DNS cache) to determine the remaining time to live
of the DNS record.
HTTP revalidation (e.g., via If-None-Match request headers) of cached
DNS information may be of limited value to DOH as revalidation
provides only a bandwidth benefit and DNS transactions are normally
latency bound. Furthermore, the HTTP response headers that enable
revalidation (such as "Last-Modified" and "Etag") are often fairly
large when compared to the overall DNS response size, and have a
variable nature that creates constant pressure on the HTTP/2
compression dictionary [RFC7541]. Other types of DNS data, such as
zone transfers, may be larger and benefit more from revalidation.
DNS API servers may wish to consider whether providing these
validation enabling response headers is worthwhile.
The stale-while-revalidate and stale-if-error cache control
directives may be well suited to a DOH implementation when allowed by
server policy. Those mechanisms allow a client, at the server's
discretion, to reuse a cache entry that is no longer fresh under some
extenuating circumstances defined in [RFC5861].
All HTTP servers, including DNS API servers, need to consider cache
interaction when they generate responses that are not globally valid.
For instance, if a DNS API server customized a response based on the
client's identity then it would not want to globally allow reuse of
that response. This could be accomplished through a variety of HTTP
techniques such as a Cache-Control max-age of 0, or perhaps by the
Vary response header.
6.2. HTTP/2
The minimum version of HTTP used by DOH SHOULD be HTTP/2 [RFC7540].
The messages in classic UDP based DNS [RFC1035] are inherently
unordered and have low overhead. A competitive HTTP transport needs
to support reordering, parallelism, priority, and header compression
to achieve similar performance. Those features were introduced to
HTTP in HTTP/2 [RFC7540]. Earlier versions of HTTP are capable of
conveying the semantic requirements of DOH but may result in very
poor performance for many uses cases.
6.3. Server Push
Before using DOH response data for DNS resolution, the client MUST
establish that the HTTP request URI is a trusted service for the DOH
query. For HTTP requests initiated by the DNS API client this trust
is implicit in the selection of URI. For HTTP server push ([RFC7540]
Section 8.2) extra care must be taken to ensure that the pushed URI
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is one that the client would have directed the same query to if the
client had initiated the request. This specification does not extend
DNS resolution privileges to URIs that are not recognized by the
client as trusted DNS API servers.
6.4. Content Negotiation
In order to maximize interoperability, DNS API clients and DNS API
servers MUST support the "application/dns-udpwireformat" media type.
Other media types MAY be used as defined by HTTP Content Negotiation
([RFC7231] Section 3.4).
7. IANA Considerations7.1. Registration of application/dns-udpwireformat Media TypeHoffman & McManus Expires October 4, 2018 [Page 10]

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To: ietf-types@iana.org
Subject: Registration of MIME media type
application/dns-udpwireformat
MIME media type name: application
MIME subtype name: dns-udpwireformat
Required parameters: n/a
Optional parameters: original_transport
The "original_transport" parameter has two defined values,
"udp" and "tcp". This parameter is only expected to be used by
servers.
Encoding considerations: This is a binary format. The contents are a
DNS message as defined in RFC 1035. The format used here is for DNS
over UDP, which is the format defined in the diagrams in RFC 1035.
Security considerations: The security considerations for carrying
this data are the same for carrying DNS without encryption.
Interoperability considerations: None.
Published specification: This document.
Applications that use this media type:
Systems that want to exchange full DNS messages.
Additional information:
Magic number(s): n/a
File extension(s): n/a
Macintosh file type code(s): n/a
Person & email address to contact for further information:
Paul Hoffman, paul.hoffman@icann.org
Intended usage: COMMON
Restrictions on usage: n/a
Author: Paul Hoffman, paul.hoffman@icann.org
Change controller: IESG
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Internet-Draft DNS Queries over HTTPS April 20188. Security Considerations
Running DNS over HTTPS relies on the security of the underlying HTTP
transport. This mitigates classic amplification attacks for UDP-
based DNS. Implementations utilizing HTTP/2 benefit from the TLS
profile defined in [RFC7540] Section 9.2.
Session level encryption has well known weaknesses with respect to
traffic analysis which might be particularly acute when dealing with
DNS queries. HTTP/2 provides further advice about the use of
compression (Section 10.6 of [RFC7540]) and padding (Section 10.7 of
[RFC7540]).
The HTTPS connection provides transport security for the interaction
between the DNS API server and client, but does not inherently ensure
the authenticity of DNS data. A DNS API client may also perform full
DNSSEC validation of answers received from a DNS API server or it may
choose to trust answers from a particular DNS API server, much as a
DNS client might choose to trust answers from its recursive DNS
resolver. This capability might be affected by the response media
type.
Section 6.1 describes the interaction of this protocol with HTTP
caching. An adversary that can control the cache used by the client
can affect that client's view of the DNS. This is no different than
the security implications of HTTP caching for other protocols that
use HTTP.
A server that is acting both as a normal web server and a DNS API
server is in a position to choose which DNS names it forces a client
to resolve (through its web service) and also be the one to answer
those queries (through its DNS API service). An untrusted DNS API
server can thus easily cause damage by poisoning a client's cache
with names that the DNS API server chooses to poison. A client MUST
NOT trust a DNS API server simply because it was discovered, or
because the client was told to trust the DNS API server by an
untrusted party. Instead, a client MUST only trust DNS API server
that is configured as trustworthy.
A client can use DNS over HTTPS as one of multiple mechanisms to
obtain DNS data. If a client of this protocol encounters an HTTP
error after sending a DNS query, and then falls back to a different
DNS retrieval mechanism, doing so can weaken the privacy and
authenticity expected by the user of the client.
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Internet-Draft DNS Queries over HTTPS April 20189. Operational Considerations
Local policy considerations and similar factors mean different DNS
servers may provide different results to the same query: for instance
in split DNS configurations [RFC6950]. It logically follows that the
server which is queried can influence the end result. Therefore a
client's choice of DNS server may affect the responses it gets to its
queries. For example, in the case of DNS64 [RFC6147], the choice
could affect whether IPv6/IPv4 translation will work at all.
The HTTPS channel used by this specification establishes secure two
party communication between the DNS API client and the DNS API
server. Filtering or inspection systems that rely on unsecured
transport of DNS will not function in a DNS over HTTPS environment.
Some HTTPS client implementations perform real time third party
checks of the revocation status of the certificates being used by
TLS. If this check is done as part of the DNS API server connection
procedure and the check itself requires DNS resolution to connect to
the third party a deadlock can occur. The use of OCSP [RFC6960]
servers or AIA for CRL fetching ([RFC5280] Section 4.2.2.1) are
examples of how this deadlock can happen. To mitigate the
possibility of deadlock, DNS API servers SHOULD NOT rely on DNS based
references to external resources in the TLS handshake. For OCSP the
server can bundle the certificate status as part of the handshake
using a mechanism appropriate to the version of TLS, such as using
[RFC6066] Section 8 for TLS version 1.2. AIA deadlocks can be
avoided by providing intermediate certificates that might otherwise
be obtained through additional requests.
A DNS API client may face a similar bootstrapping problem when the
HTTP request needs to resolve the hostname portion of the DNS URI.
Just as the address of a traditional DNS nameserver cannot be
originally determined from that same server, a DNS API client cannot
use its DNS API server to initially resolve the server's host name
into an address. Alternative strategies a client might employ
include making the initial resolution part of the configuration, IP
based URIs and corresponding IP based certificates for HTTPS, or
resolving the DNS API server's hostname via traditional DNS or
another DNS API server while still authenticating the resulting
connection via HTTPS.
HTTP [RFC7230] is a stateless application level protocol and
therefore DOH implementations do not provide stateful ordering
guarantees between different requests. DOH cannot be used as a
transport for other protocols that require strict ordering.
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